Stephan Busch Philip Bangert Slavi Dombrovski …...since launch. The SEU rate observed in a 8kB...

65th International Astronautical Congress, Toronto, Canada. Copyright ©2014 by the International Astronautical Federation. All rights reserved.

IAC-14.B4.6B.6 of 11

IAC-14.B4.6B.6

UWE-3, IN-ORBIT PERFORMANCE AND LESSONS LEARNED OF A MODULAR AND FLEXIBLE

SATELLITE BUS FOR FUTURE PICOSATELLITE FORMATIONS

Stephan Busch

University Wuerzburg, Germany, [email protected]

Philip Bangert


Slavi Dombrovski


Klaus Schilling


Formations of small satellites offer promising perspectives due to improved temporal and spatial coverage and

resolution at reasonable costs. The UWE-program addresses in-orbit demonstrations of key technologies to enable

formations of cooperating distributed spacecraft at pico-satellite level. In this context, the CubeSat UWE-3 addresses

experiments for calibration and evaluation of real-time attitude determination and control.

UWE-3 introduces also a modular and flexible pico-satellite bus as a robust and extensible base for future

missions. Technical objective was a very low power consumption of the COTS-based system, nevertheless providing

a robust performance of this miniature satellite by advanced microprocessor redundancy and fault detection,

identification and recovery software. This contribution addresses the UWE-3 design and mission results with

emphasis on the operational experiences of the attitude determination and control system.

I. INTRODUCTION

Modern miniaturisation technologies enable

realisation of satellites at continuously decreasing

masses. In an extreme case pico-satellites provide at

about 1 kg of mass a fully functional satellites. Inherent

limitations of performance and increased susceptibility

to noise effects are to be compensated by an advanced

control and filtering software, as well as by integrating

multiple distributed, cooperating pico-satellites [10, 11].

The UWE-program (University Würzburg’s

Experimental satellites) develops step-by-step the

relevant technologies for such formation flying

capabilities at pico-satellite level [12]. The first German

pico-satellite UWE-1 (launched 2005) addressed as

scientific objective the optimization of Internet Protocol

parameters to the space environment as a basis for the

future networked satellites [2]. UWE-2 (launched 2009)

focussed on attitude determination techniques, while

UWE-3 (launched 2013) extended this approach to

attitude control. Future formations of satellites use a

closed loop control in orbit to keep an appropriate

topology for efficient measurements with minimum

ground control interaction [8].

The niche market of pico-satellites exhibits dramatic

growth with about 100 pico- and nano-satellites

launched in 2013, and becomes a technology innovation

driver in order to enable robust and reliable operations

[7].

Figure 1: UWE-3 flight model after integration in Jan.

2013.

This contribution addresses at the example of

UWE-3 capabilities of the new generation of advanced

pico-satellites, offering a flexible and robust design, as

well as a performance [4, 5, 6], which supports future

operational missions. Figure 2 displays the building

blocks from which according to mission needs related

satellite designs can be flexibly assembled.

Standardized connector interfaces replace here the

traditional harness.

mailto:[email protected]




IAC-14.B4.6B.6 of 11

Figure 2: Overview of the UWE-3 modular architecture.

UWE-3 had specific emphasis on implementation of

an efficient attitude determination and control system at

minimum mass [9]. Its demonstrated in-orbit

performance will be described in details, as this will be

a crucial component for future pico-satellite formations,

promising very good potential to complement as a

distributed sensor network the traditional large, multi-

functional satellites.

II. EARLY OPERATIONS

UWE-3 was launched on Nov. 21st at 07:10:11 UTC

on board a Dnepr rocket. Within the first months of

operations all subsystems and redundancies have been

tested, both antennas have been successfully deployed

and communication with ground could be established

with both radio configurations.

From beginning of operations UWE-3 shows

excellent health state. Maximum temperatures vary

within ±30°C showing a mean temperature of +4°C.

During normal operations battery charge states remain

stable above 90%. Several radiation-induced failures

have been observed in the redundant on-board computer

since launch. The SEU rate observed in a 8kB unused

RAM region is in the order of 1 upset per month.

Various faults states of the active processing unit were

recovered by automated fail-over to the redundant

processing unit, thus ensuring continuous operation

without significant interruptions.

Operations with the attitude determination and

control systems started about a week after launch with a

long scale analysis of the satellite’s motion after

deployment. The satellite was rotating at a rate of about

23 deg/s on Dec. 3rd with its main contribution on the

body z-axis. This rotation was found to decline naturally

by about 0.5 deg/s/day until the active detumbling was

initiated on Dec. 17th. The ADCS decelerated the

satellite from 16.5 deg/s within 7 minutes of active

control to about 1 deg/s.

ADCS operations initially focused on characterizing

the attitude estimation performance, for which an in-

orbit calibration of the magnetic field sensors was the

first necessary step. With sufficiently calibrated sensors

an attitude estimation accuracy of a few degrees could

be demonstrated, rendering more advanced attitude

control experiments possible. Furthermore, a natural

alignment of the satellite with the Earth’s magnetic field

could be identified, implying the presence of a residual

magnetic dipole within the satellite. This magnetic field

following behaviour in the absence of active control was

further investigated as described in section V.I and

marked the beginning of the attitude control

experimenting phase.

First results from UWE-3 early operations and first

attitude determination and control experiments have

been published in [1] and [3].

III. AUTOMATED GROUNDSTATION

OPERATIONS

A typical operations workday of the UWE team

members begins at 7:00 UTC and ends at 16:00 UTC

(summer time). During this period three to four satellite

overpasses can be used to perform operations. However,

there are another three to four overpasses during the

night period (20:00 – 01:00 UTC) such that a remote

accessibility of the university’s ground station was

mandatory to enable operations from the outside.

The ground segment software is build up of two

main parts: ground station server and operations client.

The server is basically responsible for proper hardware

setup (e.g. antenna orientation, transceiver) and is able

to broadcast received satellite packets along the

connected clients. It transmits client packets (e.g. tele

commands, requests) via the radio and synchronizes all

clients among themselves, providing the operator an

insight into the actions of his collaborators. The client

software can be executed on arbitrary workstations

presupposed that the server is reachable within the

available network (e.g. via SSH tunnel). Typically

multiple workstations are simultaneously engaged

during the operations.

The operations process mainly consists of: receive

housekeeping data, send tele commands, request remote

files and uplink files (e.g. configuration). The

commanding interface has a wide functional range -

inter alia file system operations, ADCS controlling,

enquire and change system states, script execution etc.

During the initial operations phase all actions had to be

initiated manually as soon as the satellite was within the

range. It has been discovered that a significant amount

of time during an overpass remained unused –

especially when the operator had to react dynamically to

the current satellite’s system state (reaction time).

Consequently, many night overpasses were completely

unused.

The on board data handling system of the UWE-3

was designed in such a way as to enable the software


IAC-14.B4.6B.6 of 11

being replaced in-orbit. The size of a software image is

about 140 kB; the theoretical AFSK uplink rate is 9.600

baud with AX.25 data layer protocol on top.

Considering the occurred uplink throughput issues, the

maximum AX.25 packet size (250 bytes hardware

limitation) and the protocol overhead, the image uplink

would take up to one week of manual operations.

In order to resolve these issues and to increase the

data throughput, the operations had to be automatized.

That is to enable a predefinition of all necessary

operation tasks, which are automatically executed as

soon as the desired satellite is reachable. Furthermore,

each task can be executed periodically – e.g. daily or

during every overpass. Based on these requirements an

operations scheduler has been implemented. To allow

auto-operations without connected clients, the scheduler

is located on the ground station server.

The server has been extensively updated to fulfil the

requirements of the scheduler. That is fast orbit

prediction of a single or multiple satellites, rapid

detection of all currently set hardware settings,

straightforward exchangeability of the hardware

components (e.g. for debug purposes).

With the new software design a frequency sweep

test could be performed. During every overpass this task

was continuously adding a random offset within the

predefined range to the uplink frequency with

subsequent message uplink. Later all messages were

downlinked and mapped to the frequencies at which

they have been transmitted. This way a superior

frequency was determined at which most packets were

received.

Figure 3: Number of UWE-3 packets submitted by radio

amateurs.

Using the optimized uplink frequency an updated

software image could be transmitted with the new

uplink task. This task continuously transmits file

packets and periodically asks for chunks which need to

be transmitted again as a consequence of link errors.

The image uplink was accomplished after 7 overpasses.

A further downlink yield improvement was achieved

by involving the radio amateurs. For this purpose a web

server was introduced to enable external submission of

received UWE-3 packets. In order to automate this

process, the auto-submission was integrated into the

software which is commonly used by the radio

amateurs. This way the UWE-3 team is able to receive

packets even if the satellite is not within range. As it can

be seen in Figure 3, the amount of received packets was

drastically increased after the web server kick-off in

April 2014.

Figure 4: Receive locations of UWE-3 packets

submitted by radio amateurs.

In order to test the scheduler ability of multiple

tracking, the UNISAT-6 satellite from GAUSS-Team

was added as the additional target. In case of

simultaneous reachability of multiple targets, the one

with the higher priority is privileged. During every

overpass of UNISAT-6, a listener task is activated such

that all received packets can be saved and forwarded to

a remote server. For this purpose the GAUSS team has

established a web server with the same interface, which

has been defined for the radio-amateurs. Hereby the

new downlink-only ground station network approach

has been successfully verified. Every ground station

which is capable of multiple satellite tracking can be

easily integrated into this network. To also share the

uplink capabilities of a ground station a joint project

with the WARR team of the University of Munich

(TUM) is currently in progress. The developed ground

station server software allows simple and

straightforward hardware adaptation. Multiple servers

can be connected to a grid network, within which every

node can grant hardware access to other nodes. The

access rules are described within the centralized node

scheduler.


IAC-14.B4.6B.6 of 11

IV. ANALYSIS OF GLOBAL INTERFERENCES

IN AMATEUR RADIO BAND

UWE-3 employs two fully redundant UHF

transceivers with separate omnidirectional monopole

antennas operating in the 70cm amateur radio band

(435.000 MHz – 438.000 MHz). While the satellite’s

tranceivers provide about 1 Watt transmit power, the

universities groundstation operates at 70W on a 3m

cross-yagi antenna with 14dB gain.

From beginning on the downlink quality from the

satellite to ground was excellent, showing high signal

strenghts even at low elevations. However, the

telecommand uplink was initially difficult with

transmission failure rates between 80% and 90%. From

beginning of 2014 the uplink quality degraded further,

resulting in average failure rates between 90-95%,

which could sporadically extend to 98-100% for several

passes, while still providing excellent downlink quality.

Both, ground and space borne hardware defects

could be excluded from potential failure sources as the

problems remained when operating on the redundant

devices. The problem has been further analyzed by

performing various automated uplink tests. In order to

efficiently identify the uplink success rate, short data

frames containing a unique id have been frequently

uplinked directly to a file in the satellites flash memory

without acknowledgement requests. In the course of the

test, parameters such as offsets in antenna pointing or

uplink frequency have been altered randomly. After

downloading the file, the received frame ids could be

directly compared with the uplinked frame ids to

correlate the success rate with instantaneous frequency

offset, satellite position, etc. As it can be seen in Figure

5, a potential frequency drift, e.g. caused by temperature

variations at the satellites receiver, could also be

excluded as failure source, as the success rate is not

affected by slight frequency deviations. However, it

could be identified that the uplink quality significantly

changes in general within the amateur frequency band

and further slightly varies with the satellites relative

location.

Figure 5: Exemplary uplink quality analysis based on a

ground based frequency sweep around 435.000 MHz ±

150 kHz from 15/16 April 2014.

Following a trial-and-error search on different

frequencies, a good candidate has been found such that

the uplink quality could be improved significantly and

the satellite operations and experiments could be

conducted more efficiently again.

In beginning of June 2014 a software update has

been uploaded to one of the redundant onboard-

computers of UWE-3. Among other features, the update

included several new experiments targeting the

analyzation of the link quality. In order to further

analyze the spatial distribution of in-situ interference

levels for the whole amateur radio spectrum, an RSSI

frequency sweep experiment has been implemented.

The experiment allows to monitor and log measured

RSSI noise levels, while automatically scanning over a

specified frequency range. Beside start and stop time,

the experiment allows to configure target frequency

rage and scan interval Cf = (fstart, fstop, Δfstep). Further, the

data acquisition can be controlled with parameters for

RSSI sampling rate, number of averaged samples in a

measurement and number of repeated measurements for

a single frequency Ca = (ts, nb, nm).

As the on-board radio is used for the RSSI sampling,

first the device response to a single carrier has been

determined on the engineering model on ground. As can

be seen in Figure 6, the RSSI measurement is sensitive

to a range of fbase ± 110 kHz when the radio is

configured to a specific receive frequency fbase.

Figure 6: RSSI frequency sweep showing response to a

constant carrier signal at 437.400 MHz. Cf = (437.100

MHz, 437.800 MHz, 20 kHz), Ca = (1000 ms, 1, 3).

Figure 7 shows an in-orbit frequency sweep

recording from 15th of June when UWE-3 had a high

elevation pass-over in the west of Wuerzburg.

Throughout the entire measurement the signal emission

from our groundstation has been deactivated. It can be

seen that the background noise level increases about 5-

10 dB on both observed frequencies when the satellite

passes Europe. Further, it can be noticed that the

437.385 MHz is exposed to severe interferances over

central Europe with RSSI peak levels ranging up to -70

dBm while the 436.400 MHz is not affected by this

source. These measurements could be repeated several

times and are representative for the observed time frame

in third quarter of 2014.


IAC-14.B4.6B.6 of 11

Figure 7: RSSI noise levels [dBm] measured on-board

UWE-3 during a pass over central Europe without

groundstation operations in Wuerzburg. Cf = (436.400

MHz, 437.385 MHz, 985 kHz), Ca = (5000 ms, 1, 6).

Described observations lead to further experiments

targeting the global spatial distribution of interferences

on the entire UHF amateur radio band used for satellite

operations. Several frequency sweeps have been

recorded in third quarter of 2014, each lasting several

days to ensure global coverage. The results discussed in

the following are based on measurements recorded

between 06.08.2014 and 11.08.2014, and are

representative for the performed observations. The

recording comprises more than 100.000 data points

downloaded from the satellite in a 1.5 MB compressed

data file. The measurements have been recorded with

the following configuration: Cf = (435.000 MHz,

438.000 MHz, 200 kHz), Ca = (250 ms, 10, 2). In order

to ensure the observation of sporadic short interference

peaks, a high sampling rate of 250 ms has been chosen.

Each recorded measurement represents the average of

10 samples, thus reducing the amount of data produced.

Figure 8 shows the mean RSSI level from an exemplary

data set versus base frequency fbase and time.

Figure 8: Extract from the raw RSSI [dBm]

measurements over time [UTC]. (Colorbar similar to

Figure 9) . Cf = (435.000 MHz, 438.000 MHz, 200

kHz), Ca = (250 ms, 10, 2).

The downloaded measurements have been mapped

to their instantaneous sub-satellite points as visualized

in Figure 9. The recorded data points show sufficient

coverage of the entire globe and can properly be used as

a grid for spatial interpolation to generate a global

interference map for each frequency under

investigation. The map interpolation results are shown

in Figure 12. It can be seen that the average interference

levels vary significantly with loaction and observed

frequency. Especially orbital regions connected to

central Europe seem to be affected significantly on a

wider frequency band between 437.000 MHz and

437.600 MHz.

Figure 9: Orbit coverage of measurement recording.

In order to express these observation with concrete

numbers, a local statistical analysis has been made. For

this purpose the measurements have been filtered for

specific locations and frequencies in oder to calculate

statistical measures such as median and quartiles of the

underlying distribution of observed averaged RSSI

levels. Figure 10 depicts the statistical analysis for two

different regions. The first region is centered around the

sphere of influence around our groundstation in

Wuerzburg. The second region serves as an undisturbed

reference with a selected region of similar size located

somewhere in the south pacific (see Figure 11).

Figure 10: Average interference statistics for each

frequency for Europe centered at groundstation in

Würzburg (top) and for a reference without

interferences over the pacific (middle). (Box: first and

third quartiles, median of average RSSI (10 samples in

2.5 s); Whiskers: data in 1.5 interquartile range)

Figure 11: Reference world plots show RSSI levels

[dBm] for 437.400 MHz (corresponding to Figure 10).


IAC-14.B4.6B.6 of 11

Again, the plots clearly show the dependancy in

location and especially frequency, but it has to be

pointed out that the underlying data points represent an

average of 10 RSSI samples already. Therefore, the

extreme interference levels as present in Figure 7 are

not directly visible anymore in terms of their real

outlying value. However, it is ensured that these peaks

contribute to the statistics such that the different

frequencies can be compared with each other for

frequency selection.

The results reflect our experiences when operating

UWE-3 during the first six month after launch. They

further underline the importance of proper freqency

selection for satellites using the radio amateur bands.

Moreover, the results emphasize the requirement for in-

orbit frequency re-configuration to be able to react to

changes of inevitable global interferences caused for

example by military space surveillance radars.

435.000 MHz 435.200 MHz 435.400 MHz 435.600 MHz

435.800 MHz 436.000 MHz 436.200 MHz 436.400 MHz

436.600 MHz 436.800 MHz 437.000 MHz 437.200 MHz

437.400 MHz 437.600 MHz 437.800 MHz 438.000 MHz

Figure 12: RSSI [dBm] average interference world plot

between 06.08.2014 and 11.08.2014.

V. ADCS OPERATIONS

UWE-3 has been launched to demonstrate real-time

attitude determination and to achieve first attitude

control for the UWE platform. The former has been

accomplished, its performance characterized and the

results have been published. While working to further

improve the attitude estimation capabilities of the

satellite, the focus has shifted towards active attitude

control. Preceding work comprises the satellites active

detumbling in Dec. 2013 [1] and functionality tests of

the reaction wheel [3]. Furthermore, the presence of a

residual magntic dipole has been found, which affects

directly attitude control. In the following sections the

approach to attitude control of UWE-3 as well as

results of experiments with a spin controller are

described.

V.I Residual Magnetic Moment

Identifying certain features in the natural

movement of the satellite, it became apparent that its

rotation is dominated by the interaction of the Earth’s

magnetic field and an inherent residual magnetic

dipole µ within the satellite. In order to prepare attitude

control for absolute pointing, it was essential to

distinguish the magnetic dipole’s orientation and

strength.

High resolution recordings of the satellite’s

rotational rate show that the rate vector lies in a plane,

such that its changes also lie in the same plane. For

small rotation rates and an inertia tensor with small

off-diagonal entries, Euler’s equation simplifies to

Texternal = Tμ = μ × B = Iω̇ + ω × (Iω) ≈ Iω̇, [1]

which means that for these cases µ is perpendicular to

the plane spanned by the rotation vector.


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Figure 13: Rate vector in body coordinates. The color

code is the elapsed time. Estimated magnetic moment

is shown as red arrow.

In order to find the magnetic dipole the differential

rotation rate, �̇� was numerically found and inserted

into Euler’s equation together with an estimated inertia

tensor I and 𝜔 to give an expected torque 𝑇𝑒𝑥𝑡𝑒𝑟𝑛𝑎𝑙 in

Body coordinates. With simultaneous recordings of the

magnetic field 𝐵 in Body coordinates, a magnetic

torque 𝑇𝜇 = 𝜇 × 𝐵 can be calculated for any constant

𝜇. Using the error function 𝐸 = ∑(𝑇𝑒𝑥𝑡𝑒𝑟𝑛𝑎𝑙 − 𝑇𝜇)2, a

magnetic dipole could be found that produces a torque

which matches the experienced torque as shown in

Figure 14.

Figure 14: Torques acting upon the satellite in x-, y-,

and z-direction respectively. In blue, the total torque

derived from the satellite’s spin rate is shown. The

torque produced by the residual magnetic dipole at

every instance is shown in red.

The estimated magnetic dipole was found using

several recordings to be on the order of

𝜇 ≈ [−0.0005, +0.0115, −0.0386] 𝐴𝑚². This dipole

is shown in Figure 13 where its perpendicularity onto

the plane of 𝜔 is clearly visible. Furthermore, the

magnetic field in Body coordinates naturally swings

about this dipole, which is shown in Figure 15.

Figure 15: Magnetic Field vector in body

coordinates, the red arrow indicates the estimated

magnetic dipole.

The dipole mainly lies in the satellite’s z-axis

which limits the list of potential causes to internal

magnetic fields in the batteries, the reaction wheel and

the satellite’s antennas. During ground-testing with the

engineering model, the reaction wheel and batteries

could be eliminated which leaves the strong

assumption that the antennas, made out of stainless

steel, have been magnetized after launch.

Since the dipole is in the order of the one produced

by the magnetic torquers, measures to demagnetize the

dipole have been investigated, ranging from applying a

constant magnetic field in its opposite direction using

the torquers, to fast and random magnetic fields.

Unfortunately, none of the experiments could

significantly change the residual magnetic dipole.

However, precise attitude control appears feasible

taking spin-stabilization into account. During an

experiment with very high rotational speeds of up to 90

deg/s, the satellite’s stability against the magnetic

dipole became apparent. This is shown in Figure 16

and Figure 17, where the satellite’s angular velocity

direction in ECI coordinates is plotted. In Figure 16 the

satellite slowly at about 1 deg/s, while the rotation

vector in ECI coordinates changes direction due to the

influence of the magnetic dipole torque. Whereas in

Figure 16 the satellite spins at about 80 deg/s which

stabilizes the satellite, such that its rotation vector in

ECI coordinates shows only minor changes in direction

over the course of one orbit. During eclipse the attitude

estimation is not precise enough, such that the

gyroscopic rate cannot be transformed into ECI

coordinates and therefore no data points are shown for

this period.


IAC-14.B4.6B.6 of 11

Figure 16: Angular velocity direction in ECI

coordinates during a slow rotation of about 1 deg/s.

The rate vector changes direction in ECI coordinates

throughout the orbit, indicating that an external

torque dominates the satellite’s motion.

Figure 17: Angular velocity direction in ECI

coordinates during a fast rotation of about 80 deg/s.

The rate vector retains its direction in ECI

coordinates throughout the orbit, indicating that the

satellite’s motion is spin stabilized.

V.II High rotation rate attitude determination

Spin-stabilized attitude control poses the

requirement on the attitude determination system to be

capable of estimating the attitude even at high

rotational rates. Especially for CubeSat systems

employing low power micro-controllers as

computational platform this requirement is not easily

fulfilled. Very accurate timing of the sensors’

measurements as well as efficient algorithms are

necessary to have a reliable attitude estimation at high

rotational speeds.

The UWE-3 attitude determination has previously

been characterized [1] using the estimated orientation

and transferring the measurements into the ECI frame.

The angular difference between the reference models

and the measurements gives an estimate of the attitude

determination accuracy, still containing the sensors’

noise and therefore serving as a worst case estimate. It

was previously found to be on the order of 2.4 deg and

6.9 deg for the magnetic field and the sun-sensor

measurements, respectively (RMS deviation from

reference data).

Using the same analysis on a recording during

which the satellite was spinning at 80 deg/s with the

rate vector being 𝜔 = [73, −2, −32] deg/s, the attitude

determination accuracy is given in Table 1 and the

results are also shown in Figure 18 and Figure 19. The

deviations are slightly increased compared to the case

of a slowly spinning satellite. Especially for the sun-

sensors this increase is due to small uncertainties in

timing of the sensors.

However, it should be noted that these deviations

still contain the sensor’s internal noise. The sun-

sensors have an accuracy according to their datasheet,

of ±5 deg and the magnetometers have been shown

during tests to be precise to better than 3 deg.

Table 1: Deviations of sensor measurements from

predicted reference models while spinning at 80 deg/s.

Deviation of measurements from

Reference Magnetic Vector RMS:

2.9 deg

Deviation of measurements from

Reference Sun Vector RMS:

9.6 deg

Figure 18: Magnetic Field vector in ECI coordinates

compared to the IGRF model.


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Figure 19: Sun vector in ECI coordinates compared

to the Sun model.

V.III Detumbling

Another requirement for safe spin-stabilized

control is that the ADCS is capable of slowing down

high spin rates reliably. Its effectiveness has already

been shown in Dec. 2013 during the initial detumbling

[1]. Figure 20 shows the detumbling process from an

initial spin rate of 80 degs/s, which was completed in

about 40 minutes. Shown in Figure 21 is the

corresponding 3-D representation of the rotation rate

vector in different projections. From this it becomes

visible, that the ADCS is capable of slowing down the

satellite even at very high spin rates.

Furthermore, the algorithm stabilizes the satellite

with an RMS of 0.7353 deg/s about zero (0.3968,

0.4953, 0.3714 deg/s in x, y, z-components

respectively).

Figure 20: Rate vector during detumbling from about

80 deg/s. The satellite reaches its stable state after 40

minutes and remains with an RMS of 0.7353 deg/s

about zero afterwards.

Figure 21: 3-D representation of the angular rate

vector during detumbling.

V.IV Spin-Z Controller

Spinning up the satellite about one defined axis

precisely, has been the next step towards spin-

stabilized attitude control. In Figure 22 an experiment

is shown during which the satellite was spun up from

an initial set-point of -10 deg/s about its z-axis to +10

deg/s about its z-axis. The controller intends to

minimize the rotation about the x- and y-axes

simultaneously.

Figure 22: Angular rate vector during spin-up to

setpoint 10 deg/s from an initial setpoint of -10 deg/s

about the body z-axis.


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The controller is able to perform the transition of

20 deg/s about the z-axis within 18 minutes, after

which it retains the intended spin rates. The

corresponding 3-D trace of the angular velocity is

shown in Figure 23.

Figure 23: 3-D representation of the angular rate

vector during a spin-up experiment to setpoint [0, 0,

10] deg/s.

However, there are two periods during which the y-

axis rate significantly moves away from its set-value.

A correlation with the measured magnetic field reveals

that during this period the magnetic field along the

satellite’s z-axis vanishes (i.e. the magnetic field lies

within the xy-plane) and thus, the controller is not able

to produce efficiently torques in the xy-plane, leaving

these axes underactuated. During the rest of its

activation time the controller retains an RMS of

[0.1196, 0.25077, 0.0892] deg/s for the x-, y-, and z-

axis about their set-points respectively.

The successful test of this spin-z controller has

confirmed the effectivity of a stepwise approach to

spin stabilized attitude control. A controller allowing

an arbitrary spin axis is in preparation, targeting a spin

axis perpendicular to the residual dipole such that

effects of the latter onto the satellite’s attitude can be

minimized. Furthermore, the improved controller shall

retain the spin axis constant in ECI coordinates, such

that Sun-pointing for instance can be achieved.

Figure 24: The top shows the magnetic field’s body

z-component throughout the spin-z experiment

whereas the bottom graph shows the angular rate

vector. Note that the periods in which ωy deviates

significantly from its set-point coincide with very low

magnetic field strengths in the body z-direction,

leaving the body xy-plane underactuated.

VI. CONCLUSIONS

Pico-satellite formations offer excellent potential

for efficient applications in Earth observation and

telecommunication missions. Essential enabling

technologies related to modular, flexible system design

approaches, to robust on-board data handling systems

based on advanced FDIR-software, as well as to

miniature attitude determination and control system

implementation are addressed in the UWE-3 project.

Key subsystems and components have successfully

been demonstrated in orbit, preparing the subsequent

formation demonstration mission “NetSat”.

As an important milestone related to attitude

determination and control, an internal residual

magnetic moment could be found by analysing

gyroscopic and magnetic measurements. It could be

shown, that the ADCS is capable of reliably estimating

the satellite’s attitude even at high spin rates, enabling

spin stabilized control. Furthermore, efficient spin-up

and detumbling could be demonstrated, laying ground

for further control experiments targeting inertial spin

stabilized control.

ACKNOWLEDGEMENTS

The authors appreciated the support for UWE-3 by

the German national space agency DLR (Raumfahrt-

Agentur des Deutschen Zentrums für Luft- und

Raumfahrt e.V.) by funding from the Federal Ministry

of Economics and Technology by approval from

German Parliament with reference 50 RU 0901, as

well as the European Research Council advanced grant

“NetSat”.


IAC-14.B4.6B.6 of 11

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Stephan Busch Philip Bangert Slavi Dombrovski …...since launch. The SEU rate observed in a 8kB...

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